But the violent outburst, while shocking and distressing, was not a complete surprise; threats and aggression toward members of Congress have gotten a lot of press in recent months, notably at town hall-style events. And especially related to the health care debate.

There was a lot of talk in the immediate aftermath of the shooting about political polarization, specifically the vitriol aimed at politicians.

"In our political culture, people don't give elected officials the benefit of the doubt," said Rep. Ryan Costello, R-Penn. "They certainly don't give members of Congress the benefit of the doubt. They probably think a lot of things about me that are totally false."

"But people down here, members of Congress, are just trying to make the country better," Costello added.

A spokesman for Tenney's office confirmed the authenticity of the email to NPR, and also sent screenshots of Facebook comments on her page that talked about the possibility of her son, a Marine, being killed.

Weaver drops out

The Democrat running for Iowa's 4th Congressional District dropped out on June 3, citing threats to her safety, among other reasons.

Kim Weaver, who lost to Rep. King in the 2016 race, announced the decision on Facebook:

"After much deliberation, I have determined that the best decision for me is to withdraw my candidacy for the U.S. House race in Iowa's 4th Congressional District.

"One consideration has been raised again by recent events at my home. Beginning during my 2016 campaign, I have received very alarming acts of intimidation, including death threats. While some may say enduring threats are just a part of running for office, my personal safety has increasingly become a concern."

King sought to pin the blame on the left, and did so again after Wednesday's shooting.

"America has been divided," said King, according to The Washington Post. "And the center of America is disappearing, and the violence is appearing in the streets, and it's coming from the left."

"I was just so mad"

In May, two people were removed from a town hall meeting focused on health care held by GOP Rep. Kevin Cramer in North Dakota.

According to the Bismarck Tribune, one man in the audience of about 80 people approached the congressman, following a woman's tearful testimony about her sick child.

"After taking dollar bills out of his wallet, [resident Mike Quinn] attempted to shove them into Cramer's suit pocket ... pushing him in the process while saying his money could go toward the medical expenses of the woman's child," the Tribune reports.

In an interview with the newspaper after the event, Quinn, who was not arrested, said he was reacting to what he thought was evasion on the part of the congressman.

"It was spur of the moment. I was just so mad. I just kept getting angrier and angrier," he said. Quinn also insisted that his actions weren't planned or connected to any organization.